In recent years, from a viewpoint of preservation of global environment and effective use of natural energy, offshore wind power generation has been attracting attention. Various types of floating structures have been proposed for offshore wind power generation, including a spar type designed to have a center of gravity constantly below a center of buoyancy to ensure static stability; a TLP (tension leg platform) type designed to be moored by tensioned cables called tension legs; and a semi-submergible type designed to have a deck and a lower hull connected by columns to reduce the waterplane area (cross-sectional area at the waterline) and to reduce oscillating motions by utilizing a phase difference between wave forces acting on the submerged part. Among these types, the spar type is considered advantageous over the others in terms of costs.
Generally, the spar-type floating structure comprises a tall, thin, approximately-columnar floating body and a ballast portion provided at the bottom of the floating body so that the weight of the ballast portion allows the floating body, moored by cables, to float in upright position on the sea (see patent documents 1 and 2, for example).
For example, patent document 1 discloses a spar-type floating structure comprising an upper floating body with the waterline located on, a lower floating body greater in diameter than the upper floating body, and a ballast tank provided at the bottom of the lower floating body, and patent document 2 discloses a spar-type floating structure comprising an upper floating body with the waterline located on, a lower floating body greater in diameter than the upper floating body, and a ballast tank connected to the underside of the lower floating body by connecting steel pipes.